The present invention relates to energy or heat storage media having high heat capacity which render them potentially useful to systems with facilities for the storage of energy or heat along with requisite heat transfer equipment.
Heat storage reservoirs are provided in cases where the amount of heat supply and heat demand differ from each other within given time periods. In many instances, the heat energy is available for a short period of time, while the heat consumption is more or less continuous. The use of solar energy for heating and for industrial processes is an example of this situation. Time unsteady industrial processes would be another.
Many different energy storage media have been used in the past. Stones, rocks, concrete blocks, bricks, gravel, and the like are relatively inexpensive but have the disadvantage of a very low heat capacity. Water is one of the most commonly used media. It has a reasonably high heat capacity and good heat transfer properties. The difficulty with water is that an enormous volume of it is needed to store a reasonable amount of heat.
Metal salt hydrates or the eutectic mixture of such hydrates with other hydrates or with ion-generating compounds are also suitable as energy storage media. Examples of such energy storage substances are sodium thiosulfate or sodium phosphate when the storage media are to be used for heating purposes. Examples of storage media used for cooling purposes are sodium sulphate mixed with sodium chloride or calcium chloride and water. Crystallizing solutions have discontinuities in their cooling curves as a consequence of the change in phase from liquid to solid. Usually, during the change in phase large amounts of energy are absorbed or released and usually this phase transition occurs over a relatively narrow temperature range. Thus, the storage of energy must also take place in a narrow temperature range, usually from 25.degree. to 50.degree. C. In addition, salts present severe heat transfer problems with resultant low thermodynamic efficiency in the overall storage systems in which they are used. Finally, these salts suffer greatly from cyclical losses due to stratification.
U.S. Pat. No. 4,153,105, Reversible Latent Heat Storage Method and Reversible Latent Heat Accumulator by Johann Schroder, issued May 8, 1979, discloses the use of aqueous solutions of potassium fluoride and sodium sulphate hydrate as heat storage media. In the description of prior art in column 1 of the patent, salt hydrates are disclosed which include nucleating materials added which do not dissolve in the medium but which substantially increase the number of nuclei formed and also include the addition of an organic (for example, gelatine) or inorganic (for example, water-glass) colloidal carrier material which assists in finally dispersing the nucleating agents throughout the heat storage medium by solidifying the heat storage medium in the form of a gel. The patent goes on to say that gels of this kind age comparatively quickly and their thermal conductivity is very low. The gels which are utilized in the present invention have a long life and their thermal conductivity is relatively high.